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Old 22nd Apr 2015, 2:42 pm   #1
Ian G4XFC
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Default VFO removal in an RA17L

Hi, I have just joined this forum and read most of the posts on the RA17L. I acquired my set at the National Hamfest last year, and not really played with it yet. My introduction to the beasts was when I joined the RAF in 1965 as a Spec Op Tel so I have had plenty of experience on using them but none on repairing them.

The set I have had a faulty rectifier valve and the wrong mains lead so I hard wired it and added silicon diodes to get it working, the meter is O/C and at the moment have a temporary meter connected while I find something more suitable.

The sensitivity of the radio is rather low and on reading the posts decided to give it a full overhaul and as one of the main things to do is the VFO's I have removed the front panel and would like to know what is the easiest way of getting them off the set to refurbish and how they are fixed to the main chassis.

Ian
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Old 22nd Apr 2015, 7:50 pm   #2
g4aaw pete
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Default Re: VFO removal in an RA17L

Hello Ian

disassembly is an per section 4 of the manual.
Remove all the knobs, remove the front panel, remove the slotted screws securing the various modules of interest.

The only difficulty is in testing them outside the chassis!

Good luck
Pete
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Old 22nd Apr 2015, 10:32 pm   #3
Ian G4XFC
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Default Re: VFO removal in an RA17L

Thanks Pete I want them out to clean and check them replace what wants doing and sticking them back in. Not looking forward to it, but I love the receiver and it has to be done

Ian
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Old 23rd Apr 2015, 8:48 am   #4
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Default Re: VFO removal in an RA17L

I made these up to check d.c. conditions and 'break in' for resistance checks. Couple of valve bases, some choc-block and a couple of Cinch plugs.

They don't solve everything, but they save some grubbing about in awkward positions!
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Old 23rd Apr 2015, 4:09 pm   #5
Ian G4XFC
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Default Re: VFO removal in an RA17L

Excellent idea Russel, its not much fun grovelling about in a large radio especially of the weight of the RA17. Going to see where I can get the bits from to make them up as an ideal addition to the tool box.

On my first post, I had been studying the manual but not got as far as section 6 but now fully read, re-read and understood.

Ian
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Old 23rd Apr 2015, 5:41 pm   #6
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Default Re: VFO removal in an RA17L

Hi Peter,
you mentioned fitting silicon diodes in lieu of the failed rectifier. whilst this got you going it's not a good long term solution as the low forward voltage drop of the silicon diodes compared to a valve rectifier will result in the HT voltage being substantially higher than it should be. This will eventually cause problems because of over-stressed components, excessive heat and the like. better to spend a few quid and buy a rectifier valve.
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Old 23rd Apr 2015, 11:36 pm   #7
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Arrow Re: VFO removal in an RA17L

In response to Peter's post #6 . . . .

About 20 years ago I replaced the valve rectifier in my RA17W with silicon diodes - and have never any experienced any problems whatsoever in doing that. Having done that modification, there is a reduction in the heat generated inside the cabinet (cooler running transformer; no heat from the rectifier valve) which is a good thing, of course, since nothing causes deterioration to electronic components faster that excessive heat and damp.
45 years ago I did exactly the same modification to my Eddystone 888A using BY100 diodes. They have never failed. Again, I have never experienced any problems as a consequence of that mod. For receivers similar to those types I would unquestionably fit that modification every time.
I usually use a base from a defunct Octal valve, so that the solid-state assy. can be unplugged at any time and the appropriate valve rectifier immediately inserted into the chassis-mounted socket - reverting the design to standard - should the need or situation demand, which up to now has never occurred.

Al.
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Old 23rd Apr 2015, 11:54 pm   #8
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Default Re: VFO removal in an RA17L

Quote:
Originally Posted by Ian G4XFC View Post
. . . and would like to know what is the easiest way of getting them (the two VFOs) off the set to refurbish and how they are fixed to the main chassis.
Mechanically, the two VFOs are secured to the die-cast chassis with long 2 BA screws. To remove the second VFO, it is necessary to remove the first VFO first.
As for low sensitivity, that is usually caused by the cumulative effect of many defective resistors and capacitors and the need to accurately set up the 37.5 MHz amp. and filters. The 2nd. VFO usually requires a complete rebuild, since an inspection usually shows many cooked (and changed value) Rs and Cs. And the mixer valve in that assy. is not above reproach either, on occasion.

Al.
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Old 24th Apr 2015, 7:41 am   #9
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Default Re: VFO removal in an RA17L

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Originally Posted by Skywave View Post
Mechanically, the two VFOs are secured to the die-cast chassis with long 2 BA screws. To remove the second VFO, it is necessary to remove the first VFO first.
I got the second VFO out of mine without removing the first VFO. Once I'd de-soldered the three connections beneath, removed the cover (to get at one of the long BA screws) and removed the pluggery and the Xtal calibrator, it just lifted straight up.

Mind, it might've been easier to remove the 1st VFO first!
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Old 24th Apr 2015, 10:31 am   #10
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Default Re: VFO removal in an RA17L

Hi Guys.

Just a thing about replacing valve rectifiers with silicon.

I understood that with a silicon replacement, before the heaters have warmed up and the valves are conducting, the HT will be quite a bit higher, thus causing stress on the smoothing cap. With a valve rectifier, it heats up at a similar rate to the other valves, thus the HT will not rise quickly and cause less stress.

When BY100s first came out as a replacement for selenium rectifiers in TVs there was a 15R 5W resistor in series to limit inrush current.
Maybe this should be a consideration too.
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Old 24th Apr 2015, 10:21 pm   #11
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Arrow Re: VFO removal in an RA17L

Hello Wendy - Yes, I'm glad you mentioned that since subsequent to my last post I have had a look through my maintenance records. I see that the inclusion of a surge limiting resistor in series with each diode is indeed something that I have done. I see that I have also fitted a series RC network across the primary winding of the mains transformer to act as a spike suppressor.

Al. / April 24, '15 /\
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Old 25th Apr 2015, 1:20 pm   #12
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Default Re: VFO removal in an RA17L

Quote:
Originally Posted by Skywave View Post
About 20 years ago I replaced the valve rectifier in my RA17W with silicon diodes - and have never any experienced any problems whatsoever in doing that. Having done that modification, there is a reduction in the heat generated inside the cabinet (cooler running transformer; no heat from the rectifier valve) which is a good thing, of course, since nothing causes deterioration to electronic components faster that excessive heat and damp.
45 years ago I did exactly the same modification to my Eddystone 888A using BY100 diodes. They have never failed. Again, I have never experienced any problems as a consequence of that mod. For receivers similar to those types I would unquestionably fit that modification every time.
I usually use a base from a defunct Octal valve, so that the solid-state assy. can be unplugged at any time and the appropriate valve rectifier immediately inserted into the chassis-mounted socket - reverting the design to standard - should the need or situation demand, which up to now has never occurred
Now that is a strange coincidence - my remarks on the dangers of substituting silicon diodes for the valve rectifier are based on my experience with an RA17W!!
When I bought the receiver it was a non-runner (and fitted with silicon diodes in an Octal valve base exactly as Al describes) The HT smoothing capacitor was swollen so I changed it along with a selection of resistors which had changed in value, but the real problem was insulation breakdown in the IF transformers in the crystal and LC filter sections of the IF Amplifier. These are specific to the RA17W and I have never found any spares. The insulation breakdown provides a path to chassis from HT of about 4kohm - low enough to upset the operation of the circuit. I have never come across this problem before in any of the Ra17/117's I have owned, and so decided the problem was caused by the HT over-voltage caused by the use of silicon diodes.
The receiver still doesn't work - if anybody has a spare RA17W IF strip I would love to hear about it! - and while I can't prove it, I am convinced the excess Ht was the cause. My opinion for what it's worth.
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Old 25th Apr 2015, 4:02 pm   #13
Ian G4XFC
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Default Re: VFO removal in an RA17L

Thanks for all the comments, I have used silicon diodes before and have always added a small value limiting resistor to compensate for the increased voltage.

My set looks untouched and I did manage to finaly remove VFO 2 and to me it seems silly to have to remove the screening shield which results in a fiddly removal of the last screw on the shield, which I will replace with one that has a hex head on it as a spanner would certainly be better than a slotted screw in that location.

I plan now to order replacements for the Hunts caps and check all the resistors and replace where necessary. Is there a cheap supplier of polyester caps in the UK?

Ian
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Old 25th Apr 2015, 11:39 pm   #14
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Quote:
Originally Posted by G8BBZ View Post
The receiver still doesn't work . . . if anybody has a spare RA17W IF strip, I would love to hear about it!
You can substitute the IF strip from an RA-17L for that in an RA-17W. The differences are the user-selectable bandwidths and those differences, in practice, are not very significant. The other difference between the L and the W relates to the user-selectable AVC time-constant: again, not a critical change.

Al.
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Old 26th Apr 2015, 11:14 am   #15
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Default Re: VFO removal in an RA17L

I could indeed fit an RA17L IF strip but then the receiver would be an RA17L and not an RA17W. I already have several RA17L's and the attraction of the W is the different IF bandwidths etc. As the W is in good shape cosmetically, I will hang on and see if I can fix it.

Thanks to wendymott and you for the input on using series resistors with silicon diodes and a snubber network on the transformer primary - good one to remember.
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